merge a series of level dbs to provide an output db of a specified length

leveldb_merge.pl

#!/usr/bin/env perl
use strict;
use warnings;
use Tie::LevelDB;

sub foreach_key {
  my ($db_name, $block) = @_;
  my $db = new Tie::LevelDB::DB($db_name);
  my $it = $db->NewIterator;

  for ($it->SeekToFirst; $it->Valid; $it->Next) {
    return 1 if $block->($it->key, $it->value) == 0;
  }

  return 0;
}

unless (caller) {
  my ($count, $output,  @dbs) = @ARGV;
  
  if (!$count || !$output) {
    print STDERR "usage: <output count> <output db name> [source...]\n";
    exit 2;
  }

  my $out_db = new Tie::LevelDB::DB($output);
  my $current = 0;

  foreach my $db (@dbs) {
    print "working with: $db\n";

    my $ret = foreach_key($db, my $s = sub {
      my ($k, $v) = @_;
      $out_db->Put($k, $v);

      return (++$current == $count) ? 0 : 1;
    });

    if ($ret == 1) {
      print "finished - enough data!\n";
      last;;
    }
  }

}